1. Field of the Invention
This invention relates generally to glass manufacturing processes and, more particularly, to glass manufacturing processes incorporating one or more optical low-coherence interferometry (OLCI) systems to determine a thickness or thickness profile of a glass ribbon. In one exemplary aspect, the invention also relates to a float glass system incorporating one or more optical low-coherence interferometry (OLCI) systems to determine a coater gap with respect to a coater located in a float bath.
2. Technical Considerations
In a conventional float glass process, glass batch materials are melted in a furnace to form a glass melt. The glass melt is poured onto the top of a pool of molten metal, typically molten tin, at the entrance end of a float bath. The glass melt spreads out over the top of the molten tin to form a glass ribbon. This glass ribbon is stretched and pulled by mechanical devices in the float bath to provide the glass ribbon with a desired uniform thickness or a desired thickness profile (i.e., contour or thickness variation across the width of the ribbon). The glass ribbon exits the float bath and can be transported to a lehr for controlled cooling to strengthen or temper the glass, if desired.
While in the float bath, one or more coatings can be applied onto the top of the glass ribbon by a conventional chemical vapor deposition (CVD) coating process. In this in-bath CVD coating process, vaporized coating materials are transported to one or more coaters positioned in the float bath above the glass ribbon. The coating materials exit the bottom of the coater(s) and are deposited on top of the glass ribbon to form a coating. The structure and operation of a conventional float glass process as well as that of a conventional CVD coating process will be well understood by one of ordinary skill in the art and, therefore, will not be described in detail.
During the coating process in the float bath, the coater gap, i.e., the distance between the bottom of the CVD coater and the top of the hot float glass ribbon, is important for the coating process. This distance impacts the color uniformity of the resultant coating and also the thickness of the coating. Further, this coater gap is important for the safety of the coater, which could be damaged if the coater accidentally contacts the underlying hot glass ribbon. In a conventional float glass system, the in-bath CVD coater is typically only about 0.2 inches (0.5 centimeters) above the top of the hot glass ribbon, which can be on the order of about 1,400° F. (760° C.).
In most conventional float glass systems, the distance of the CVD coater above the glass ribbon is set or adjusted by an operator relying on visual observation and past coating experience. Typically, the operator looks through a window on the side of the float bath and judges whether the coater gap he observes is correct based on his practice and experience. If he determines that the coater gap is incorrect or needs adjustment, the operator uses a movement system connected to the coater to raise or lower the coater and then visually reassesses whether the new coater gap looks correct. Further, the uniformity of the coater gap across the coating area (i.e. the parallelism between the bottom of the coater and the top of the glass ribbon) is important. If the coater is tilted with respect to the top of the glass ribbon, this can adversely impact the coating process and the resultant coating and could lead to coater damage if a portion of the coater accidentally contacts the hot glass ribbon.
Additionally, the thickness of the glass ribbon is important. The desired glass ribbon thickness depends upon the final use of the glass being made and must be within certain tolerances for the glass to be commercially acceptable for its intended purpose. The glass ribbon thickness is dependent upon such factors as the rate of addition of the glass melt into the float bath and the travel speed of the glass ribbon through the float bath. Therefore, the operators of the glass furnace and/or the float bath need to know whether the thickness of the glass ribbon exiting the float bath is within the specified limits for the final product. However, the glass ribbon thickness as it exits the float bath is difficult to measure due to the high temperature of the glass ribbon, the flexibility of the hot glass ribbon, and the fact that the glass ribbon is typically tilted as it exits the float bath. It is particularly difficult to accurately measure the thickness of glass ribbons greater than 10 millimeters thick. If the glass ribbon thickness is out of specification, the resultant glass sheets cannot be used for their intended purpose, thus decreasing the productivity of the float glass process. By reducing the time the thickness of the glass ribbon is out of specification, the yield of the float glass process can be increased.
Glass can also be formed using a downdraw process, in which the glass ribbon moves vertically downwardly under the force of gravity as it cools. Examples of downdraw processes include the slot downdraw process, in which molten glass flows out of a slot below the glass furnace to form a glass ribbon; and the fusion (or overflow) downdraw process in which molten glass overflows the opposed sides of a forming trough and the two glass films fuse below the trough to form a glass ribbon. In a downdraw process, as in a float glass process, the thickness of the glass ribbon is an important factor.
It would be desirable to provide a more convenient and accurate way of determining the glass ribbon thickness in a glass manufacturing process that reduces or eliminates at least some of the problems associated with known processes. For example, it would be desirable for float bath operators to have a less subjective way of setting the coater gap in a float glass system to prevent accidental contact of the coater with the glass ribbon and/or to improve the coating process. For example, it would be desirable to provide a more convenient and accurate way of determining the glass ribbon thickness either in the float bath and/or after the glass ribbon exits from the float bath. For example, it would be desirable to provide a glass ribbon thickness measurement system for glass ribbons thicker than about 10 mm. For example, it would be desirable to provide a system that could not only simplify the setting of the coater gap but which also allows for determining the glass ribbon thickness and/or the thickness of a coating on the glass ribbon. For example, it would be desirable to provide a more convenient and accurate way of determining the glass ribbon thickness in a downdraw glass manufacturing process.